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Identification of the endothelial cell binding site for factor IX
W F Cheung1, J van den Born, K Kühn
1Department of Medical and Physiological Chemistry, University of Uppsala, Sweden.
Summary
Researchers identified collagen IV as the binding site for factor IX on endothelial cells. Specific mutations in factor IX affected its binding affinity, supporting collagen IV as the endothelial cell receptor.
Area of Science:
- Biochemistry
- Cell Biology
- Hematology
Background:
- The amino terminus (residues 3-11) of factor IX is crucial for binding to endothelial cells.
- Mutations like factor IX K5A and factor IX V10K abolish this binding.
- Factor IX K5R mutation enhances endothelial cell binding affinity.
Purpose of the Study:
- To identify the specific binding site for factor IX on bovine aortic endothelial cells.
- To investigate the role of collagen IV as a potential binding partner for factor IX.
Main Methods:
- Competitive binding assays using 125I-labeled factor IX.
- Immobilization of tetrameric collagen IV on microtiter plates.
- Analysis of binding affinities (Kd) for wild-type factor IX and mutant forms (factor IX K5R).
Main Results:
- Factor IX and factor IX K5R competed with labeled factor IX for binding to immobilized collagen IV.
- Factor X, factor VII, and non-binding factor IX mutants (K5A, V10K) did not compete for binding.
- Dissociation constants (Kd) for factor IX binding to collagen IV were determined (6.8 nM for wild-type, 1.1 nM for K5R).
Conclusions:
- Collagen IV is identified as a strong candidate for the factor IX binding site on endothelial cells.
- The binding affinity of factor IX to collagen IV correlates with its affinity to endothelial cells.
- Further studies are needed to determine the physiological relevance of this interaction.